Abstract: Excessive application of phosphate fertilizer in agriculture leads to the loss of unused phosphorus, which poses a serious threat to water quality. Therefore, identifying key areas of phosphorus loss in agricultural fields as well as their influencing factors is crucial for effective pollution management. Although the Phosphorus Index Model (PIM) has been widely applied and developed to evaluate phosphorus loss risks and identify key source areas in small watersheds, research on such risk assessments in wheat-maize rotation field at different scales remains limited. In this study, a phosphorus index evaluation system was constructed using phosphorus surplus from wheat-maize rotation field and soil available phosphorus as source factors, and surface runoff, distance to rivers, and distance to the lake as transport factors. The phosphorus loss risks and key nfluencing factors in wheat-maize rotation field in different years (2015, 2017 and 2019) and scales (sources, transporting to rivers and transporting to the lake) were examined, focusing on the core area of the Baiyangdian Basin, specifically Baoding, Dingzhou, and Xiong’an New Area. The results showed that during the study period, the key risk areas of phosphorus losses were primarily concentrated in the northeastern and southwestern agricultural regions of the study area, and the proportion of extremely high-risk zones to total in 2015 was 2.70%, with all extremely high-risk zones converted into high-risk or blow zones in 2019. The key risk areas for phosphorus transporting to rivers were more prevalent along both sides of the rivers, and the proportion of extremely high-risk zones to total in 2015 was 0.95%, with all extremely high-risk zones area converted into high-risk or blow zones in 2019. The key risk areas for phosphorus transporting to the lake were primarily located in the southwestern and northeastern counties surrounding the target water body, and the proportion of extremely high-risk zones to total decreased from 3.30% in 2015 to 0.29% in 2019. The key factors influencing phosphorus transporting to the lake included phosphorus surplus from wheat-maize rotation field and transport factors (e.g., surface runoff and distance to the lake), both of which were significantly correlated with phosphorus loss risk (P<0.05 or P<0.01). Phosphorus surplus from wheat-maize rotation field, primarily sourced from chemical fertilizer (69.32%). These findings provide a scientific reference for assessing phosphorus loss risk in the Baiyangdian Basin and offer guidance for the effective management of agricultural non-point source pollution.